Role of cobalt additive on formation and anticorrosion properties of PEO coatings on AA2024 alloy in alkali-silicate electrolyte


Mechanisms of PEO coatings formation on AA2024 aluminum alloy in an aqueous solution, containing 3 g/L NaOH and 5 g/L Na2O∙2.9SiO2∙18H2O (base electrolyte) and with addition of 1.5 g/L CoOOH∙H2O, are studied. This addition results in a formation of black coatings. It is determined that structure, phase and elemental compositions of the coatings correlated with the corrosion properties. For coatings, grown in the base electrolyte, anodic and cathodic current densities and the value of impedance modulus (|Z|f=0.01Hz) improves with the increase of the coatings thickness due to the healing of internal pores during extended PEO processing, according to the results of electrochemical measurements in 0.5 wt% NaCl solution. However, the results of electrochemical tests of coatings, synthesized in the electrolyte with CoOOH∙H2O differ. For short-term immersion (up to 24 h), thin coating (~17 μm) demonstrates the lowest current densities and the highest value of impedance modulus (|Z|f=0.01Hz) in comparison to the thicker coatings (~48, 82 μm). However, after longer exposure (up to 168 h) the thickest coating (~82 μm) has the best long-term stability. This behavior and the most probable mechanism of black coatings formation are suggested and explained as function of coating properties and exposure time in NaCl solution.
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